A Highly Delocalized Triplet Carbene, 5-Methylhexa-1,2,4-triene-1,3-diyl: Matrix IR Identification, Structure, and Reactions

Ethynylhydroxycarbene (H-C≡C-C̈-OH)

The species on the C₃H₂O potential energy surface have long been known to play a vital role in extraterrestrial chemistry. Here we report on the hitherto uncharacterized isomer ethynylhydroxycarbene (H-C≡C-C̈-OH, 1) generated by high-vacuum flash pyrolysis of ethynylglyoxylic acid ethyl ester and trapped in solid argon matrices at 3 and 20 K. Upon irradiation at 436 nm trans-1 rearranges to its higher lying cis-conformer. Prolonged irradiation leads to the formation of propynal. When the matrix is kept in the dark, 1 reacts within a half-life of ca. 70 h to propynal in a conformer-specific [1,2]H-tunneling process. Our results are fully consistent with computations at the CCSD(T)/cc-pVTZ and the B3LYP/def2-QZVPP levels of theory.

Infrared-Spectroscopic Study of (4-Methylpent-3-en-1-ynyl)methylthiocarbene, Its Photochemical Transformations, and Reactions in an Argon Matrix

The first representative of singlet carbenes bearing both ethynyl and methylthio groups at the carbene center, (4-methylpent-3-en-1-ynyl)methylthiocarbene, has been generated in a low-temperature Ar matrix upon UV photolysis of 3,3-dimethyl-5-methylthioethynyl-3H-pyrazole and detected by FTIR spectroscopy. The generation of the carbene proceeds via intermediate (3-diazo-5-methylhex-4-en-1-ynyl)methylsulfane. The comparison of FTIR spectroscopy data with the results of quantum chemical calculations (B3LYP/aug-cc-pVTZ) and NRT analysis shows that (4-methylpent-3-en-1-ynyl)methylthiocarbene has a singlet ground state with the localization of the unpaired spins on the carbon atom in the α-position to methylthio moiety. Two major pathways of further phototransformation of the studied carbene have been found. One of them produces photochemically stable thioketone (S═CMe-C≡C-CH═CMe₂) as a result of methyl group migration from sulfur to the neighboring carbon atom, and the other one leads to the formation of labile thioketene (S═C═C(Me)-CH═CH-CMe═CH₂). Ability of (4-methylpent-3-en-1-ynyl)methylthiocarbene to insert into the H-Cl bond was established, which additionally confirms the singlet nature of this intermediate.

Using N-Heterocyclic Vinyl Ligands to Access Stable Divinylgermylenes and a Germylium Cation

Two efficient methods are presented to install σ- and π-electron-donating N-heterocyclic vinyl groups onto main-group elements (E): halosilane elimination and base-induced E-C bond formation. Placement of two NHC=CH^- ligands (NHC=N-heterocyclic carbene) onto a Ge^II center affords a two-coordinate germylene, a heavy congener of the elusive divinyl carbenes. The π-donating ability of this vinylic ligand scaffold was further demonstrated by the synthesis of a three-coordinate germylium cation R₃ Ge⁺ .

Synthesis of vinyl germylenes

Reaction of a disilylated germylene phosphane adduct with phenylacetylene unexpectedly occurs stereo- and regioselectively as insertion into the Si–Ge bond.

Preparation of surface-attached polymer layers by thermal or photochemical activation of α-diazoester moieties

We report on the attachment of polymer monolayers or surface-attached, polymer networks onto SiO2 and/or polymer surfaces using thermo- and photoreactive α-diazoester groups. In the prior case, the α-diazoester groups are introduced into the system in the form of self-assembled monolayers of appropriately functionalized silanes. The monolayer decorated substrates are coated by polymer films and the α-diazoester groups in the monolayer are activated by heat or irradiation with UV-light. Upon activation, they cleave off nitrogen and the resulting carbene intermediates insert into C-H bonds of neighboring polymer chains. As a result of this binding process, surface-attached monolayers of the deposited polymer are obtained. When the polymers themselves carry such reactive moieties, the photo- or thermal activation leads to cross-linking of the polymers and thin surface-attached polymer networks result from the same process. The formation of the surface-attached layer is studied as a function of activation conditions, especially the temperature and the wavelength of the light used in the process.

Recent developments of cyclopropene chemistry

This critical review discusses recent developments in the field of cyclopropene chemistry. Although several excellent reviews that mainly focused on the thermolysis and pyrolysis as well as metal-mediated reactions of cyclopropenes have been published, significant new developments have also been achieved in recent years. This brand new review provides an overview of the progress from 2007 to 2011 on the syntheses and transformations of cyclopropenes as well as their related mechanistic studies (238 references).